An accelerometer (acceleration sensor) includes a seismic or proof mass on a spring. When the accelerometer experiences an external force such as gravity, the proof mass is displaced until the external force is balanced by the spring force. The displacement is translated into an electrical signal proportional to acceleration.
Modern accelerometers are often small micro electro-mechanical systems (MEMS), with certain designs based on a proof mass suspended by one or more cantilever beams. Under the influence of external accelerations the proof mass deflects from its neutral position. This deflection is measured using some transduction method. Most commonly, the capacitance between a set of fixed beams and a set of beams attached to the proof mass is measured. This method is simple, reliable, and inexpensive, but requires signal conditioning close to the sensing parts. Integrating piezoresistors in the springs to detect spring deformation, and thus deflection, is a good alternative, although a few more process steps are needed during the fabrication sequence. For very high sensitivities quantum tunneling is also used; this requires a dedicated process making it very expensive. Optical measurement has been demonstrated on laboratory scale.
Another, far less common, type of MEMS-based accelerometer contains a small heater at the bottom of a very small dome, which heats the air inside the dome to cause it to rise. A thermocouple on the dome determines where the heated air reaches the dome and the deflection off the center is a measure of the acceleration applied to the sensor.
Most micro electro-mechanical accelerometers operate in-plane, that is, they are designed to be sensitive only to a direction in the plane of the die. By integrating two devices perpendicularly on a single die a two-axis accelerometer can be made. By adding an additional out-of-plane device three axes can be measured. Such a combination always has a much lower misalignment error than three discrete models combined after packaging.
Micromechanical accelerometers are available in a wide variety of measurement ranges, reaching up to thousands of G's and beyond. By way of illustration, and without limitation, high G sensors can achieve a measurement range of at least 10,000G. For high G sensors, the design should include a damping mechanism (damper) to properly damp or suppress the motion or ringing of the proof mass and its supporting frame, which, for a non-limiting example, can be achieved with squeeze-film damping. The designer must make a compromise between sensitivity and the maximum acceleration that can be measured. There are other, basic, economic restraints on acceleration sensor design. For non-limiting examples, small sensor size is important both for cost and for application of the sensor and the means of fabrication of the sensor must be established.
There exist ways to adapt an existing low-range acceleration sensor to a higher range one. One approach is to stiffen the measuring structure (e.g., the frame to support the proof mass) of the sensor, but such stiffening would quickly outrun both the available damping of the proof mass and its coupling to the damper of the sensor. Another approach is to diminish the area of the proof mass. However, reducing the area of the proof mass also diminishes the coupling to the damper linearly, and the damping coefficient and effectiveness of the damper more strongly than linearly.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent upon a reading of the specification and a study of the drawings.